This invention relates to a device for forming a simulated stereophonic sound field, i.e., a device for forming a sound field having a conversion circuit converting a monophonic signal to a stereophonic signal.
Known in the art is a conversion circuit as shown in FIG. 1 which converts a monophonic signal to a stereophonic signal. In this circuit, two twin-T circuits T.sub.1, T.sub.2 are cascade-connected and a monophonic signal is applied to an input terminal 1. A signal H(S) outputted by the twin-T circuits T.sub.1, T.sub.2 is derived from an output terminal 2 as a right channel signal whereas a signal 1-H(S) which is obtained by subtracting the output signal H(S) of the twin-T circuits T.sub.1, T.sub.2 from the input signal in a subtractor 3 is derived from an output terminal 4 as a left channel signal.
FIG. 2 shows frequency characteristics of the right channel signal H(S) and the left channel signal 1-H(S). These signals H(S) and 1-H(S) have frequency characteristics which are complementary to each other. More specifically, frequency components in the vicinity of frequencies f.sub.1 and f.sub.2 are reproduced mainly from the left channel and frequency components below the frequency f.sub.1, above f.sub.2 and in the vicinity of a peak frequency f' between the frequencies f.sub.1 and f.sub.2 are reproduced mainly from the right channel. Such separation of the frequency band into the right and left channels in reproduction of sound can simulate a stereophonic reproduction. For example, assuming that f.sub.1 =150 Hz and f.sub.2 =4.5 kHz, f' becomes about 1 kHz in which case bass is reproduced mainly from the left channel and soprano mainly from the right channel.
In the circuit of FIG. 1, however, the output signal H(S) which has passed the twin-T circuits T.sub.1, T.sub.2 is attenuated, due to filtering operations of both circuits T.sub.1 and T.sub.2, in a section between the two null points as shown in FIG. 2. If, for example, the frequencies f.sub.1 and f.sub.2 are set at the above described values, the level between the two null points drops by about 12 dB. Likewise, the left channel signal 1-H(S) obtained by subtracting the right channel signal H(S) from the input signal also has its peak level deviated from 0 dB. Accordingly, the respective peak values in frequency characteristics of the signals H(S) and 1-H(S) of the respective channels do not match with one another at 0 dB so that imbalance occurs in the frequency band.
There has also been known a device as shown in FIG. 3 which converts 2-channel stereophonic signals to 4-channel stereophonic signals in a simulated fashion. Left and right channel signals of 2-channel stereophonic signals are applied to input terminals 10 and 12. These left and right channel signals are applied directly to front left and right loudspeakers 24 and 26 through power amplifiers 16 and 22. The left and right channel signals are also applied to a reverberation imparting circuit 32 to produce sounds imparted with an artificial echo and the output signals of the reverberation imparting circuit 32 are applied to rear left and right channel loudspeakers 28 and 30 through power amplifiers 18 and 20. According to this construction, since sounds produced from the front loudspeakers are the input 2-channel stereophonic signals themselves, little error is produced in localization and a sound free from unnaturalness can be obtained. This type of prior art circuit however has the disadvantages that the sound produced tends to lack in feeling of presence when the imparting of reverberation is not suitably made, that unnaturalness occurs in the sound produced from the rear loudspeakers and that use of a delay circuit in the reverberation imparting circuit makes the construction of the circuit complicated and realization of reduced distortion factor difficult.